This invention relates to an automotive brake fluid pressure control device that can perform automatic braking functions such as traction control and vehicle stability control.
For economical reasons, the most popular automotive antilock brake control systems are what is known as the circulation type, which has a wheel brake fluid pressure control valve unit including a discharge valve and provided in a main fluid line extending from a master cylinder to each wheel brake, a fluid reservoir for temporarily storing brake fluid discharged through the discharge valve, and a power pump for sucking fluid in the reservoir and returning it into the main fluid line.
Various braking systems have been proposed which are versions of the circulation type and can perform automatic braking such as traction control and vehicle stability control.
One of them is disclosed in Japanese patent publication 64-74153, which includes a main line, a fluid supply line and an on-off valve adapted to connect the fluid supply line to the main line only during traction control, and a pump provided in the fluid supply line for applying brake fluid to the wheel brake during traction control. The on-off valve opens only during traction control and otherwise kept shut. While the on-off valve is closed, there is no spare fluid in the fluid supply line. Thus, when traction control starts, brake fluid has to be sucked to the pump from the master cylinder reservoir through the master cylinder, brake piping, and the on-off valve. Thus, high suction resistance makes it impossible to apply brake fluid to the wheel brake quickly upon start of the traction control.
Japanese Patent Publication 5-116607 disclosed an improved brake fluid pressure control system, which was proposed to solve these problems.
This unit is basically a circulation type antilock brake control system with the following elements added: that is, a fluid supply line branching from the main fluid line at its point between the master cylinder and a fluid return point at which the line from the pump outlet merges: a discharged fluid reservoir provided at the end of the fluid supply line: an on-off valve for checking fluid flow from the fluid supply line to the discharged fluid reservoir during traction control: an intermediate fluid reservoir provided in the fluid supply line for supplying fluid during traction control.
The applicant of the present invention also proposed a system having a similar intermediate reservoir in unexamined Japanese patent publication 8-108838. In either of these systems, fluid can be more smoothly supplied to the pump inlet from the intermediate reservoir during automatic braking control than with a system without an intermediate reservoir. But since these reservoirs are both nonactive ones and thus fluid is sucked under the atmospheric pressure, it is difficult to smoothly supply fluid to the pump inlet so that the drake pressure can be increased with sufficient quickness necessary for vehicle stability control particularly when the ambient temperature is low and the fluid viscosity is high.
Japanese patent publication 4-231241 proposes to use a power-driven active pressure accumulator instead of the above-described (passive) fluid reservoir. Although such an active accumulator greatly improves the responsiveness and the fluid supply capability, such a system is naturally very costly because the accumulator needs an actuator and an actuator circuit.
Unexamined Japanese patent publication 11-59377 proposes a system having a means for storing fluid at a higher-than-atmopheric pressure. This system needs a sealing mechanism for maintaining the higher-than-atmopheric pressure.
One problem with this system is that failure of the sealing mechanism leads to leakage of fluid. This considerably increases the brake pedal stroke.
Also, it is practically impossible to completely prevent leak of brake fluid stored at a higher-than-atmospheric pressure. Thus, fluid tends to be lost gradually. One solution to this problem is the provision of means for restoring fluid during braking. But when this means is activated, the brake pedal stroke increases.
An object of the invention is to provide a brake fluid pressure control unit that has improved reliability and improved pedal feeling.